In early drilling practice wells were drilled as near to the vertical as possible. Later it became common to drill directional or slanted wells to gain access to hydrocarbon deposits located underneath ground sites where it was not feasible to set up a drilling rig. As oil and gas exploration and production moved into offshore areas, it became conventional in view of economic considerations to drill a large number of directional boreholes from a single platform. Each well extends downward for a certain distance and then is kicked out on an inclined path that eventually reaches a target in the production zone. A downhole motor that operates in response to circulation of drilling fluid down the drill string is commonly used to rotate the bit in sections of the borehole where a change in direction is made. More recently, wells are being drilled that have a lower portion which extends horizontally in order to intersect a series of oil or gas bearing vertical fractures and thereby increase dramatically the production from a single well. In each circumstance where a directional borehole is to be drilled, there is a pressing need for precise and continuous control over the direction in which the borehole is to proceed so that a specified underground target can be reached as quickly and economically as possible. As used herein, the term "direction" means inclination with respect to the vertical, and the azimuth of such inclination.
Prior systems for controlling the direction of a borehole that is drilled using a downhole motor have employed either a rigid bent sub or bent housing to provide a permanent bend angle in the drill string above the bit, or a surface adjustable bent sub or housing which requires a round trip of the drill string in order to produce, change, or eliminate the bend angle. Such systems also have included undersized stabilizers, one located near the bit and another on top of the motor, to achieve a change in the trajectory of the borehole in a "sliding" mode, that is, where the drill string is not rotated but merely slides down the hole as the bit chips away at the rock. It also is known to superimpose drill string rotation at the surface along with downhole bit rotation from the motor with the aim of causing the bit to drill straight ahead. During rotation of the pipe, the bend point orbits about the longitudinal axis of the borehole, and although the bit wobbles slightly in this mode its overall tendency is to drill straight ahead in the same direction.
However, such prior systems have suffered from a number of significant problems. Rotation of a permanently bent or deflected motor housing can create excessive surface torque and cause cyclic bending stresses in the housing which can cause serious damage. The use of undersize stabilizers near the bit and above the motor, a requirement in sliding drilling wherein the pipe is not being rotated, frequently results in a drop in the inclination of the borehole when the pipe is rotated beyond that which is to be expected for a particular bottom hole assembly. Another problem with prior assemblies is that excessive drill string vibrations are generated by rotation of a bent bottom hole assembly that significantly reduce the useful lives to be expected of the downhole components, and which is believed to trigger certain borehole instability problems such as sloughing walls that can cause sticking of the downhole assembly.
Optimum directional control requires surface availability, substantially in real time, of certain information about the bottom section of the borehole such as its inclination, azimuth and the tool face angle. Within the past decade, apparatus and techniques have been developed for continuously measuring, while drilling, various characteristic properties of the earth formations intersected by a well bore, as well as other downhole parameters, and for telemetering the results of the measurements to the surface. Some of the parameters that can be measured and transmitted to the surface are components of the earth's gravity and magnetic fields from which the inclination with respect to vertical, azimuth with respect to magnetic North, and tool face angle can be computed, displayed, and recorded. The inclination and azimuth values can be plotted at regular depth intervals to enable a record to be made of the exact path taken by the borehole. These measurements also provide the basis for altering the path if it is not proceeding according to plan. However, the downhole means by which path corrections have been made have left much to be desired, and for one thing necessitated removing the drill string to temporarily place a special bent sub therein, or to rearrange the spacing of stabilizers.
An object of the present invention is to provide new and improved methods and apparatus combinations for use in controlling the direction of a borehole.
Another object of the present invention is to provide new and improved methods of directional drilling control that include adjusting downhole a mechanism that establishes a bend angle in the drill string to cause a change in direction.
Another object of the present invention is to provide a new and improved directional drilling control system where information relating to the orientation with respect to vertical or to North of a plane containing the axes of the bend angle below the bend point is transmitted to the surface substantially in real time.
Another object of the present invention is to provide a new and improved directional control system including bend angle and stabilizer mechanisms that can be adjusted downhole to permit more precise control over the trajectory of the bit.